Preheat the oven to 350°F (175°C). Grease and flour a 9x13 inch baking dish.

In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, pumpkin pie spice, and salt.

In another bowl, mix the pumpkin puree, vegetable oil, and eggs until well combined.

Gradually add the wet ingredients to the dry ingredients, mixing until just combined.

Pour the batter into the prepared baking dish and smooth the top.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

In a separate bowl, combine the sliced apples with cinnamon and a bit of sugar if desired.

Spread the apple mixture evenly over the baked pumpkin spice cake.

In another bowl, mix oats, brown sugar, flour, and melted butter until crumbly.

Sprinkle the oat mixture over the apples.

Return the dish to the oven and bake for an additional 15-20 minutes, or until the topping is golden brown.